The objective is to determine the nature and incidence of pathologic lesions that develop throughout the lifespan of HSR, HSR-stroke prone, and WKY rats. In each strain, 100 animals of both sexes will be selected for long-term holding. Some subjects will be killed at the following ages: 2 months, 8 months, 12 months, 15 months, 18 months, 21 months, and 28 months. Tissues will be studied by light microscopic methods, and if indicated, electron microscopic techniques. These strains have great potential as animal models for the study of cerebrovascular and hypertensive disease in human beings. Their value depends on accurate pathological description and documentation. In each strain, 100 animals of both sexes will be selected for long-term holding.